1. Field of the Invention
The present invention relates to a method for forming an insulating film layer on a semiconductor substrate surface. More particularly, the present invention relates to a method for forming an insulating film layer, which exhibits an excellent stability in electrical properties and a high degree of density, on a surface of a semiconductor substrate useful for the production of integrated circuits.
2. Description of the Prior Art
In the production of semiconductor devices, it is known that a portion of the semiconductor surface, in which a PN junction is exposed to an ambient atmosphere, is coated with an insulating film layer in order to prevent changes in the electrical properties of the semiconductor device with the lapse of time and to enhance the reliability of the semiconductor device. In the production of MISFET (metal insulator semiconductor field effect transistor) or MIS type integrated circuits, it is also known that a gate insulating film layer is formed in the semiconductor device, so as to introduce an inversion layer channel into the device. Generally speaking, in the conventional MISFET and MIS type integrated circuits, the semiconductor substrate, the insulating film layer and the metal gate electrode respectively consist essentially of silicon (Si), silicon oxide (SiO.sub.2) and aluminum (Al). However, the above-mentioned structure of the conventional metal insulator semiconductor device exhibits a disadvantage in that an application of a field of about 10.sup.6 V/cm onto the insulating film layer at a temperature of about 200.degree. C. results in a remarkable fluctuation of a gate threshold voltage. This disadvantageous phenomenon may be due to the fact that the drift effect of impurity ions contained in the SiO.sub.2 insulating film layer or the structural defects of the SiO.sub.2 insulating film layer per se cause the creation of a capture level of carriers, and the creation of the capture level of carriers results in a remarkable fluctuation in the space charge distribution in the surface layer of the semiconductor device. It is clear that the fluctuation in the space charge distribution is influenced most remarkably by the structural defects in the interface between the Si substrate and the insulating film layer (SiO.sub.2). The structural defects are remarkably created when the insulating film layer (SiO.sub.2) is formed by thermally oxidizing the surface layer of the Si substrate. This thermal oxidization process tends to permit the impurity ions, such as alkali metal ions, to be contaminated in the oxidized silicon film layer. In order to eliminate the above-mentioned defects from the conventional insulating film layer, an attempt was made to provide an insulating film layer by way of a chemical vapor deposition or sputtering. However, all of the previous attempts failed to reduce the structural defects in the interface between the Si substrate and the SiO.sub.2 layer to a level lower than that of the thermal oxidation.
Furthermore, it is known that when the surface of the Si substrate is thermally oxidized in an extremely clean atmosphere, the resultant SiO.sub.2 film layer exhibits very little structural defects in a level, in terms of surface charge density, of 10.sup.11 /cm.sup.2 or less. However, the interface between the Si substrate and the SiO.sub.2 film layer still exhibits structural defects due to excessive silicon ions. Accordingly, it has been strongly desired to eliminate the above-mentioned defects from the interface between the Si substrate and the SiO.sub.2 film layer.
On the other hand, recently, the increase in density of the integrated circuits is accompanied with a tendency to decrease the thickness of the gate insulating film layer in the semiconductor device. When a thinned insulating film layer is used, it is important that the resultant semiconductor device exhibits an excellent stability in electrical properties.
As a method for decreasing the variation in the electrical properties of the semiconductor device, it was attempted to provide a gate insulating film layer consisting of a silicon oxide film formed on a silicon substrate and a phospho-silicate glass (PSG) film formed on the silicon oxide film. However, in the case where the PSG film has a high concentration of phosphorus, an application of a high field onto the gate insulating film layer results in polarization of the gate insulating film layer. This polarization causes the potential of the semiconductor surface to be undesirably fluctuated. Also, it was found that the reduction in the thickness of the silicon oxide film causes the operation of the resultant N channel enhancement type field effect transistor to be more difficult due to the diffusion of donor impurities from the polysilicon gate film into the silicon substrate.
In another method, it was tried to utilize a silicon nitride film or aluminum oxide film as a gate insulating film layer. However, since the above-mentioned film layers are provided by means of chemical vapor deposition, structural defects are locally created in the interface between the silicon substrate and the insulating film layer. These structural defects cause the electrical properties of the semiconductor device to be unstable.
Moreover, it was attempted to utilize a silicon oxynitride film layer the properties of which are intermediate between those of the silicon oxide film layer and the silicon nitride film layer, as the gate insulating film layer. For example, the conventional silicon oxynitride film layer was produced by bringing a mixed gas containing monosilane (SiH.sub.4) gas, ammonia (NH.sub.3) gas and carbon dioxide (CO.sub.2) gas into contact with a surface of a silicon substrate heated at a temperature of from 700.degree. to 1000.degree. C. so as to react the monosilane with the ammonia and carbon dioxide in vapor phase. However, the insulating film layer produced by the above-mentioned chemical vapor deposition sometimes exhibits structural defects in the interface between the silicon substrate and the insulating film layer. Otherwise, it is sometimes difficult to produce a silicon oxynitride film layer having an even structure and composition. The above-mentioned structural defects or the structural unevenness cause the electrical properties of the semiconductor device to be unstable.